A WLAN basically supports a BSS (Basic Service Set) consisting of an AP (Access Point), which serves as the point of access of a DS (Distributed System), and a plurality of wireless STAs (STAtions) other than the AP.
The MAC (Medium Access Control) protocol of the WLAN operates based on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance). Therefore, the WLAN involves resource wasting in the course of channel contention. In order to alleviate this problem, IEEE 802.11 Workgroup ‘e’ has defined an enhanced MAC protocol, which proposes that, when a radio resource transmission right is acquired, multiple MPDUs are transmitted using SIFS (Short Inter-Frame Space) during a TXOP (Transmission Opportunity), and block ACKs are received in response thereto (i.e. burst transmission).
Furthermore, IEEE 802.11 Workgroup ‘n’ has defined an A-MSDU (Aggregated MSDU) and an A-MPDU (Aggregated MDPU). At least one MSDU (which is a transmission unit) or at least one MPDU is aggregated even without IFS, and is transmitted in the process of one-time radio resource contention.
Recent surge in the number of WLAN users is followed by standardization regarding VHT (Very High Throughput) WLAN systems by IEE 802.11 Workgroup ‘ac’ as an attempt to increase data throughput provided by each BSS.
A VHT WLAN system supports, in a multiple (three)-STA environment consisting of one AP and two STAs, maximum throughput of 1 Gbps at the MAC SAP of the AP, as well as maximum throughput of 500 Mbps at the MAC SAP of a wireless STA for point-to-point environments. There is also consideration made so that each AP and STA of the VHT WLAN simultaneously supports compatibility with existing WLAN (IEEE 802.11a/n system).
When a wireless STA acquires a TXOP in a wireless communication system (e.g. WLAN), the STA needs to receive a response through a response frame regarding a request frame in order to improve reliability of wireless communication. Examples include a CTS frame responding to a RTS frame and an ACK frame responding to a transmitted data frame.
The responses are classified into immediate responses and delayed responses. Responses (ACK frames) to a single piece of data correspond to the immediate responses; and block responses responding to a continuous transmission or aggregated MPDU correspond to both immediate and delayed responses.
An immediate response is used in the following manner: when PHY-RXEND.primitive of a received request frame is generated, a response frame is transmitted after SIFS so that other wireless STAs do not transmit. In this case, the frame exchange sequence constitutes a pair, as described above. The generated response frame may not include a transmission address.
On the other hand, a delayed response is used in the following manner: a response is made through an ACK frame as a basic response for informing of whether an initially generated request frame has been received or not, and a response frame including requested information is then transmitted. The response frame in this case may be transmitted through EDCA (Enhanced Distributed Channel Access) regarding channels, piggybacked by another frame, or aggregated and transmitted together with another frame. According to the delayed response scheme, a transmitting STA receives a response frame from a receiving STA and then informs the receiving STA that the response frame has been received using an ACK frame.
The frame exchange sequence in such a WLAN may be applied to a multiple-user wireless communication system. When uplink MU-MIMO technology is supported, wireless STAs can receive frames according to the above-mentioned frame exchange sequence and simultaneously transmit response frames after IFS.
When uplink MU-MIMO technology is not supported, or when better throughput is desired even if uplink MU-MIMO technology is supported, a wireless STA, after receiving a frame, needs to transmit a response frame using a difference of channel or time.
Schemes for exchanging frames using a difference of time but the same bandwidth in a WLAN system include a scheme of exchanging a request frame eliciting response frames and a response frame through channel access for each wireless STA, and a scheme of transmitting, by a plurality of wireless STAs, successive response frames through a single request frame.
To be specific, the scheme of exchanging a request frame eliciting response frames and a response frame through channel access for each wireless STA is as follows: an AP transmits a request frame to a first wireless STA through channel access; the first wireless STA transmit a response frame after SIFS; the AP transmits another request frame to a second wireless STA through channel access; and the second wireless STA transmits a response frame after SIFS.
The scheme of transmitting, by a plurality of wireless STAs, successive response frames through a single request frame is as follows: a single request frame including address information regarding multiple users is transmitted through channel access, and the wireless STAs then successively transmit response frames after SIFS.
Downlink operation in a wireless communication system based on MU-MIMO largely includes five phases, specifically a sounding phase for beamforming used in MU-MIMO, a group control phase capable of controlling transmission and reception with regard to multiple users, a NAV distribution phase for protecting MU-MIMO data from legacy wireless STAs, a beamformed data transmission phase for transmitting MU-MIMO data to multiple users within a designated group, and a response phase for receiving an ACK frame from a wireless STA that has received data.
FIG. 1 schematically illustrates downlink operation including a NAV distribution phase for informing of channel use for protecting MU-MIMO data from legacy wireless STAs.
The frame that delivers a NAV value should be decodable by legacy wireless STAs as well. For NAV distribution, when performing channel access to acquire a TXOP, wireless STAs use RTS and CTS frames or short data and ACK frame exchange, or use CTS-to-Self frames.
Similarly, it is also possible to use RTS and CTS frame exchange or CTS-to-Self frames so that legacy wireless STAs can receive in a VHT WLAN system as well.
Therefore, there is a need for a method for efficiently protecting MU-MIMO data by exchanging RTS and CTS frames in a VTH WLAN system, standardization of which is currently in progress.